The Botanic Garden Volume I Part 24

These siliceous sand-rocks are either held together by a siliceous cement, or have a greater or less portion of clay in them, which in some acts as a cement to the siliceous crystals, but in others is in such great abundance that in burning them they become an imperfect porcelain and are then used to repair the roads, as at Chesterfield in Derbyshire; these are called argillaceous grit by Mr. Kirwan. In other places a calcareous matter cements the crystals together; and in other places the siliceous crystals lie in loose strata under the marl in the form of white sand; as at Normington about a mile from Derby.

The lowest beds of siliceous sand-stone produced from mora.s.ses seem to obtain their acid from the mora.s.s, and their calcareous base from the limestone on which it rests; These beds possess a siliceous cement, and from their greater purity and hardness are used for course grinding- stones and scyth stones, and are situated on the edges of limestone countries, having lost the other strata of coals, or clay, or iron, which were originally produced above them. Such are the sand-rocks inc.u.mbent on limestone near Matlock in Derbyshire. As these siliceous sand-rocks contain no marine productions scattered amongst them, they appear to have been elevated, torn to pieces, and many fragments of them scattered over the adjacent country by explosions, from fires within the mora.s.s from which they have been formed; and which dissipated every thing inflammable above and beneath them, except some stains of iron, with which they are in some places spotted. If these sand-rocks had been acc.u.mulated beneath the sea, and elevated along with the beds of limestone on which they rest, some vestiges of marine sh.e.l.ls either in their siliceous or calcareous state must have been discerned amongst them.

2. SILICEOUS TREES.

In many of these sand-rocks are found the impressions of vegetable roots, which seem to have been the most unchangeable parts of the plant, as sh.e.l.ls and shark's teeth are found in chalk-beds from their being the most unchangeable parts of the animal. In other instances the wood itself is penetrated, and whole trees converted into flint; specimens of which I have by me, from near Coventry, and from a gravel-pit in Shropshire near Child's Archal in the road to Drayton. Other polished specimens of vegetable flints abound in the cabinets of the curious, which evidently shew the concentric circles of woody fibres, and their interstices filled with whiter siliceous matter, with the branching off of the knots when cut horizontally, and the parallel lines of wood when cut longitudinally, with uncommon beauty and variety. Of these I possess some beautiful specimens, which were presented to me by the Earl of Uxbridge.

The colours of these siliceous vegetables are generally brown, from the iron, I suppose, or manganese, which induced them to crystallize or to fuse more easily. Some of the cracks of the wood in drying are filled with white flint or calcedony, and others of them remain hollow, lined with innumerable small crystals tinged with iron, which I suppose had a share in converting their calcareous matter into siliceous crystals, because the crystals called Peak-diamonds are always found bedded in an ochreous earth; and those called Bristol-stones are situated on limestone coloured with iron. Mr. F. French presented me with a congeries of siliceous crystals, which he gathered on the crater (as he supposes) of an extinguished volcano at Cromach Water in c.u.mberland. The crystals are about an inch high in the shape of dogtooth or calcareous spar, covered with a dark ferruginous matter. The bed on which they rest is about an inch in thickness, and is stained with iron on its undersurface. This curious fossil shews the trans.m.u.tation of calcareous earth into siliceous, as much as the siliceous sh.e.l.ls which abound in the cabinets of the curious. There may sometime be discovered in this age of science, a method of thus impregnating wood with liquid flint, which would produce pillars for the support, and tiles for the covering of houses, which would be uninflammable and endure as long as the earth beneath them.

That some siliceous productions have been in a fluid state without much heat at the time of their formation appears from the vegetable flints above described not having quite lost their organized appearance; from sh.e.l.ls, and coralloids, and entrochi being converted into flint without loosing their form; from the bason of calcedony round Giesar in Iceland; and from the experiment of Mr. Bergman, who obtained thirteen regular formed crystals by suffering the powder of quartz to remain in a vessel with fluor acid for two years; these crystals were about the size of small peas, and were not so hard as quartz. Opusc. de Terra Silicea, p.

33. Mr. Achard procured both calcareous and siliceous crystals, one from calcareous earth, and the other from the earth of alum, both dissolved in water impregnated with fixed air; the water filtrating very slowly through a porous bottom of baked clay. See Journal de Physique, for January, 1778.

3. AGATES, ONYXES, SCOTS-PEBBLES.

In small cavities of these sand-rocks, I am informed, the beautiful siliceous nodules are found which are called Scot's-pebbles; and which on being cut in different directions take the names of agates, onyxes, sardonyxes, &c. according to the colours of the lines or strata which they exhibit. Some of the nodules are hollow and filled with crystals, others have a nucleus of less compact siliceous matter which is generally white, surrounded with many concentric strata coloured with iron, and other alternate strata of white agate or calcedony, sometimes to the number of thirty.

I think these nodules bear evident marks of their having been in perfect fusion by either heat alone, or by water and heat, under great pressure, according to the ingenious theory of Dr. Hutton; but I do not imagine, that they were injected into cavities from materials from without, but that some vegetables or parts of vegetables containing more iron or manganese than others, facilitated the compleat fusion, thus destroying the vestiges of vegetable organization, which were conspicuous in the siliceous trees above mentioned. Some of these nodules being hollow and lined with crystals, and others containing a nucleus of white siliceous matter of a looser texture, shew they were composed of the materials then existing in the cavity; which consisting before of loose sand, must take up less s.p.a.ce when fused into a solid ma.s.s.

These siliceous nodules resemble the nodules of iron-stone mentioned in note on Canto II. l. 183, in respect to their possessing a great number of concentric spheres coloured generally with iron, but they differ in this circ.u.mstance, that the concentric spheres generally obey the form of the external crust, and in their not possessing a chalybeate nucleus.

The stalact.i.tes formed on the roofs of caverns are often coloured in concentric strata, by their coats being spread over each other at different times; and some of them, as the cupreous ones, possess great beauty from this formation; but as these are necessarily more or less of a cylindrical or conic form, the nodules or globular flints above described cannot have been constructed in this manner. To what law of nature then is to be referred the production of such numerous concentric spheres? I suspect to the law of congelation.

When salt and water are exposed to severe frosty air, the salt is said to be precipitated as the water freezes; that is, as the heat, in which it was dissolved, is withdrawn; where the experiment is tried in a bowl or bason, this may be true, as the surface freezes first, and the salt is found at the bottom. But in a fluid exposed in a thin phial, I found by experiment, that the extraneous matter previously dissolved by the heat in the mixture was not simply set at liberty to subside, but was detruded or pushed backward as the ice was produced. The experiment was this: about two ounces of a solution of blue vitriol were accidentally frozen in a thin phial, the gla.s.s was cracked and fallen to pieces, the ice was dissolved, and I found a pillar of blue vitriol standing erect on the bottom of the broken bottle. Nor is this power of congelation more extraordinary, than that by its powerful and sudden expansion it should burst iron sh.e.l.ls and coehorns, or throw out the plugs with which the water was secured in them above one hundred and thirty yards, according to the experiments at Quebec by Major Williams. Edinb.

Transact. Vol. II. p. 23.

In some siliceous nodules which now lie before me, the external crust for about the tenth of an inch consists of white agate, in others it is much thinner, and in some much thicker; corresponding with this crust there are from twenty to thirty superinc.u.mbent strata, of alternately darker and lighter colour; whence it appears, that the external crust as it cooled or froze, propelled from it the iron or manganese which was dissolved in it; this receded till it had formed an arch or vault strong enough to resist its further protrusion; then the next inner sphere or stratum as it cooled or froze, propelled forwards its colouring matter in the same manner, till another arch or sphere produced sufficient resistance to this frigoriscent expulsion. Some of them have detruded their colouring matter quite to the centre, the rings continuing to become darker as they are nearer it; in others the chalybeate arch seems to have stopped half an inch from the centre, and become thicker by having attracted to itself the irony matter from the white nucleus, owing probably to its cooling less precipitately in the central parts than at the surface of the pebble.

A similar detrusion of a marly matter in circular arches or vaults obtains in the salt mines in Cheshire; from whence Dr. Hutton very ingeniously concludes, that the salt must have been liquified by heat; which would seem to be much confirmed by the above theory. Edinb.

Transact. Vol. I. p. 244.

I cannot conclude this account of Scots-pebbles without observing that some of them on being sawed longitudinally asunder, seem still to possess some vestiges of the cylindrical organization of vegetables; others possess a nucleus of white agate much resembling some bulbous roots with their concentric coats, or the knots in elm-roots or crab- trees; some of these I suppose were formed in the manner above explained, during the congelation of ma.s.ses of melted flint and iron; others may have been formed from a vegetable nucleus, and retain some vestiges of the organization of the plant.

4. SAND OF THE SEA.

The great abundance of siliceous sand at the bottom of the ocean may in part be washed down from the siliceous rocks above described, but in general I suppose it derives its acid only from the vegetable and animal matter of mora.s.ses, which is carried down by floods or by the atmosphere, and becomes united in the sea with its calcareous base from sh.e.l.ls and coralloids, and thus a.s.sumes its crystalline form at the bottom of the ocean, and is there intermixed with gravel or other matters washed from the mountains in its vicinity.

5. CHERT, OR PETROSILEX.

The rocks of marble are often alternately intermixed with strata of chert, or coa.r.s.e flint, and this in beds from one to three feet thick, as at Ham and Matlock, or of less than the tenth of an inch in thickness, as a mile or two from Bakewell in the road to Buxton. It is difficult to conceive in what manner ten or twenty strata of either limestone or flint, of different shades of white and black, could be laid quite regularly over each other from sediments or precipitations from the sea; it appears to me much easier to comprehend, by supposing with Dr. Hutton, that both the solid rocks of marble and the flint had been fused by great heat, (or by heat and water,) under immense pressure; by its cooling or congealing the colouring matter might be detruded, and form parallel or curvilinean strata, as above explained.

The colouring matter both of limestone and flint was probably owing to the flesh of peculiar animals, as well as the siliceous acid, which converted some of the limestone into flint; or to some strata of sh.e.l.l- fish having been overwhelmed when alive with new materials, while others dying in their natural situations would lose their fleshy parts, either by its putrid solution in the water or by its being eaten by other sea- insects. I have some calcareous fossil sh.e.l.ls which contain a black coaly matter in them, which was evidently the body of the animal, and others of the same kind filled with spar instead of it. The Labradore stone has I suppose its colours from the nacre or mother-pearl sh.e.l.ls, from which it was probably produced. And there is a stratum of calcareous matter about six or eight inches thick at Wingerworth in Derbyshire over the iron-beds, which is replete with sh.e.l.ls of fresh- water muscles, and evidently obtains its dark colour from them, as mentioned in note XVI. Many nodules of flint resemble in colour as well as in form the sh.e.l.l of the echinus or sea-urchin; others resemble some coralloids both in form and colour; and M. Arduini found in the Monte de Pancrasio, red flints branching like corals, from whence they seem to have obtained both their form and their colour. Ferber's Travels in Italy, p. 42.

6. NODULES OF FLINT IN CHALK-BEDS.

As the nodules of flint found in chalk-beds possess no marks of having been rounded by attrition or solution, I conclude that they have gained their form as well as their dark colour from the flesh of the sh.e.l.l-fish from which they had their origin; but which have been so compleatly fused by heat, or heat and water, as to obliterate all vestiges of the sh.e.l.l, in the same manner as the nodules of agate and onyx were produced from parts of vegetables, but which had been so completely fused as to obliterate all marks of their organization, or as many iron-nodules have obtained their form and origin from peculiar vegetables.

Some nodules in chalk-beds consist of sh.e.l.ls of echini filled up with chalk, the animal having been dissolved away by putrescence in water, or eaten by other sea-insects; other sh.e.l.ls of echini, in which I suppose the animal's body remained, are converted into flint but still retain the form of the sh.e.l.l. Others, I suppose as above, being more completely fused, have become flint coloured by the animal flesh, but without the exact form either of the flesh or sh.e.l.l of the animal. Many of these are hollow within and lined with crystals, like the Scot's-pebbles above described; but as the colouring matter of animal bodies differs but little from each other compared with those of vegetables, these flints vary less in their colours than those above mentioned. At the same time as they cooled in concentric spheres like the Scot's-pebbles, they often possess faint rings of colours, and always break in conchoide forms like them.

This idea of the production of nodules of flint in chalk-beds is countenanced from the iron which generally appears as these flints become decomposed by the air; which by uniting with the iron in their composition reduces it from a vitrescent state to that of calx, and thus renders it visible. And secondly, by there being no appearance in chalk- beds of a string or pipe of siliceous matter connecting one nodule with another, which must have happened if the siliceous matter, or its acid, had been injected from without according to the idea of Dr. Hutton. And thirdly, because many of them have very large cavities at their centres, which should not have happened had they been formed by the injection of a material from without.

When sh.e.l.ls or chalk are thus converted from calcareous to siliceous matter by the flesh of the animal, the new flint being heavier than the sh.e.l.l or chalk occupies less s.p.a.ce than the materials it was produced from; this is the cause of frequent cavities within them, where the whole ma.s.s has not been completely fused and pressed together. In Derbyshire there are ma.s.ses of coralloid and other sh.e.l.ls which have become siliceous, and are thus left with large vacuities sometimes within and sometimes on the outside of the remaining form of the sh.e.l.l, like the French millstones, and I suppose might serve the same purpose; the gravel of the Derwent is full of specimens of this kind.

Since writing the above I have received a very ingenious account of chalk-beds from Dr. MENISH of Chelmsford. He distinguishes chalk-beds into three kinds; such as have been raised from the sea with little disturbance of their strata, as the cliffs of Dover and Margate, which he terms _intire_ chalk. Another state of chalk is where it has suffered much derangement, as the banks of the Thames at Gravesend and Dartford.

And a third state where fragments of chalk have been rounded by water, which he terms _alluvial_ chalk. In the first of these situations of chalk he observes, that the flint lies in strata horizontally, generally in distinct nodules, but that he has observed two instances of solid plates or strata of flint, from an inch to two inches in thickness, interposed between the chalk-beds; one of these is in a chalk-bank by the road side at Berkhamstead, the other in a bank on the road from Chatham leading to Canterbury. Dr. Menish has further observed, that many of the echini are crushed in their form, and yet filled with flint, which has taken the form of the crushed sh.e.l.l, and that though many flint nodules are hollow, yet that in some echini the siliceum seems to have enlarged, as it pa.s.sed from a fluid to a solid state, as it swells out in a protuberance at the mouth and a.n.u.s of the sh.e.l.l, and that though these sh.e.l.ls are so filled with flint yet that in many places the sh.e.l.l itself remains calcareous. These strata of nodules and plates of flint seem to countenance their origin from the flesh of a stratum of animals which perished by some natural violence, and were buried in their sh.e.l.ls.

7. ANGLES OF SILICEOUS SAND.

In many rocks of siliceous sand the particles retain their angular form, and in some beds of loose sand, of which there is one of considerable purity a few yards beneath the marl at Normington about a mile south of Derby. Other siliceous sands have had their angles rounded off, like the pebbles in gravel-beds. These seem to owe their globular form to two causes; one to their attrition against each other, when they may for centuries have lain at the bottom of the sea, or of rivers; where they may have been progressively acc.u.mulated, and thus progressively at the same time rubbed upon each other by the dashing of the water, and where they would be more easily rolled over each other by their gravity being so much less than in air. This is evidently now going on in the river Derwent, for though there are no limestone rocks for ten or fifteen miles above Derby, yet a great part of the river-gravel at Derby consists of limestone nodules, whose angles are quite worn off in their descent down the stream.

There is however another cause which must have contributed to round the angles both of calcareous and siliceous fragments; and that is, their solubility in water; calcareous earth is perpetually found suspended in the waters which pa.s.s over it; and the earth of flints was observed by Bergman to be contained in water in the proportion of one grain to a gallon. Kirwan's Mineralogy, p. 107. In boiling water, however, it is soluble in much greater proportion, as appears from the siliceous earth sublimed in the distillation of fluor acid in gla.s.s vessels; and from the basons of calcedony which surrounded the jets of hot water near mount Heccla in Iceland. Troil on Iceland. It is probable most siliceous sands or pebbles have at some ages of the world been long exposed to aqueous steams raised by subterranean fires. And if fragments of stone were long immersed in a fluid menstrum, their angular parts would be first dissolved, on account of their greater surface.

Many beds of siliceous gravel are cemented together by a siliceous cement, and are called breccia; as the plumb-pudding stones of Hartfordshire, and the walls of a subterraneous temple excavated by Mr.

Curzon, at Hagley near Rugely in Staffordfshire; these may have been exposed to great heat as they were immersed in water; which water under great pressure of superinc.u.mbent materials may have been rendered red- hot, as in Papin's digester; and have thus possessed powers of solution with which we are unacquainted.

8. BASALTES AND GRANITES.

Another source of siliceous stones is from the granite, or basaltes, or porphyries, which are of different hardnesses according to the materials of their composition, or to the fire they have undergone; such are the stones of Arthur's-hill near Edinburgh, of the Giant's Causway in Ireland, and of Charnwood Forest in Leicestershire; the uppermost stratum of which last seems to have been cracked either by its elevation, or by its hastily cooling after ignition by the contact of dews or snows, and thus breaks into angular fragments, such as the streets of London are paved with; or have had their angles rounded by attrition or by partial solution; and have thus formed the common paving stones or bowlers; as well as the gravel, which is often rolled into strata amid the siliceous sand-beds, which are either formed or collected in the sea.

In what manner such a ma.s.s of crystallized matter as the Giant's Causway and similar columns of basaltes, could have been raised without other volcanic appearances, may be a matter not easy to comprehend; but there is another power in nature besides that of expansile vapour which may have raised some materials which have previously been in igneous or aqueous solution; and that is the act of congelation. When the water in the experiments above related of Major Williams had by congelation thrown out the plugs from the bomb-sh.e.l.ls, a column of ice rose from the hole of the bomb six or eight inches high. Other bodies I suspect increase in bulk which crystallize in cooling, as iron and type-metal. I remember pouring eight or ten pounds of melted brimstone into a pot to cool and was surprized to see after a little time a part of the fluid beneath break a hole in the congealed crust above it, and gradually rise into a promontory several inches high; the basaltes has many marks of fusion and of crystallization and may thence, as well as many other kinds of rocks, as of spar, marble, petrosilex, jasper, &c. have been raised by the power of congelation, a power whose quant.i.ty has not yet been ascertained, and perhaps greater and more universal than that of vapours expanded by heat. These basaltic columns rise sometimes out of mountains of granite itself, as mentioned by Dr. Beddoes, (Phil.

Transact. Vol. Lx.x.x.) and as they seem to consist of similar materials more completely fused, there is still greater reason to believe them to have been elevated in the cooling or crystallization of the ma.s.s. See note XXIV.

NOTE XX.--CLAY.

_Whence ductile Clays in wide expansion spread, Soft as the Cygnet's down, their snow-white bed._

CANTO II. l. 277.

The philosophers, who have attended to the formation of the earth, have acknowledged two great agents in producing the various changes which the terraqueous globe has undergone, and these are water and fire. Some of them have perhaps ascribed too much to one of these great agents of nature, and some to the other. They have generally agreed that the stratification of materials could only be produced from sediments or precipitations, which were previously mixed or dissolved in the sea; and that whatever effects were produced by fire were performed afterwards.

There is however great difficulty in accounting for the universal stratification of the solid globe of the earth in this manner, since many of the materials, which appear in strata, could not have been suspended in water; as the nodules of flint in chalk-beds, the extensive beds of sh.e.l.ls, and lastly the strata of coal, clay, sand, and iron-ore, which in most coal-countries lie from five to seven times alternately stratified over each other, and none of them are soluble in water. Add to this if a solution of them or a mixture of them in water could be supposed, the cause of that solution must cease before a precipitation could commence.

1. The great ma.s.ses of lava, under the various names of granite, porphyry, toadstone, moor-stone, rag, and slate, which const.i.tute the old world, may have acquired the stratification, which some of them appear to possess, by their having been formed by successive eruptions of a fluid ma.s.s, which at different periods of antient time arose from volcanic shafts and covered each other, the surface of the interior ma.s.s of lava would cool and become solid before the superinc.u.mbent stratum was poured over it; to the same cause may be ascribed their different compositions and textures, which are scarcely the same in any two parts of the world.

2. The stratifications of the great ma.s.ses of limestone, which were produced from sea-sh.e.l.ls, seem to have been formed by the different times at which the innumerable sh.e.l.ls were produced and deposited. A colony of echini, or madrepores, or cornua ammonis, lived and perished in one period of time; in another a new colony of either similar or different sh.e.l.ls lived and died over the former ones, producing a stratum of more recent sh.e.l.l over a stratum of others which had began to petrify or to become marble; and thus from unknown depths to what are now the summits of mountains the limestone is disposed in strata of varying solidity and colour. These have afterwards undergone variety of changes by their solution and deposition from the water in which they were immersed, or from having been exposed to great heat under great pressure, according to the ingenious theory of Dr. Hutton. Edinb.

Transact. Vol. I. See Note XVI.

3. In most of the coal-countries of this island there are from five to seven beds of coal stratified with an equal number of beds, though of much greater thickness, of clay and sandstone, and occasionally of iron- ores. In what manner to account for the stratification of these materials seems to be a problem of greater difficulty. Philosophers have generally supposed that they have been arranged by the currents of the sea; but considering their insolubility in water, and their almost similar specific gravity, an acc.u.mulation of them in such distinct beds from this cause is altogether inconceiveable, though some coal-countries bear marks of having been at some time immersed beneath the waves and raised again by subterranean fires.